Each remembered trace is a string that is extended as
optimization progresses and appends data to it. The
optimizer_trace_max_mem_size
variable sets a limit on the total amount of memory used by
all currently remembered traces. If this limit is reached, the
current trace is not extended (and thus is incomplete), and
the MISSING_BYTES_BEYOND_MAX_MEM_SIZE
column shows the number of bytes missing from the trace.

INSUFFICIENT_PRIVILEGES

If a traced query uses views or stored routines that have
SQL SECURITY with a value of
DEFINER, it may be that a user other than
the definer is denied from seeing the trace of the query. In
that case, the trace is shown as empty and
INSUFFICIENT_PRIVILEGES has a value of 1.
Otherwise, the value is 0.